Electrochemical energy storage systems, such as batteries, supercapacitors and the like, have been widely proposed for large-scale energy storage applications. Various battery designs, including flow batteries, have been considered for this purpose. Compared to other types of electrochemical energy storage systems, flow batteries can be advantageous, particularly for large-scale applications, due to their ability to decouple the parameters of power density and energy density from one another.
Flow batteries generally include negative and positive active materials in corresponding electrolyte solutions, which are flowed separately across opposing faces of a membrane or separator in an electrochemical cell containing negative and positive electrodes. The flow battery is charged or discharged through electrochemical reactions of the active materials that occur inside the two half-cells. As used herein, the terms “active material,” “electroactive material,” “redox-active material” or variants thereof synonymously refer to a material that undergoes a change in oxidation state during operation of a flow battery or like electrochemical energy storage system (i.e., during charging or discharging).
Although flow batteries hold significant promise for large-scale energy storage applications, they have often been plagued by sub-optimal energy storage performance (e.g., round trip energy efficiency) and limited cycle life, among other factors. Despite significant investigational efforts, no commercially viable flow battery technologies have yet been developed. Certain issues leading to poor energy storage performance, limited cycle life, and other performance-degrading factors are discussed hereinafter.
One issue occurring commonly during operation of flow batteries is that the active material concentration in one or more of the electrolyte solutions can change over time. Although degradation of an active material could result in a concentration decrease, the more common manner in which the active material concentration can change is through gain or loss of solvent. In an aqueous electrolyte solution, for example, loss of water can lead to an increase in the concentration of the active material. Water loss from an aqueous electrolyte solution can occur through a variety of means during operation of a flow battery such as, for example, due to heating during electrochemical reactions and/or when venting to release a gas generated during parasitic reactions, which are described further herein. In some cases, an aqueous electrolyte solution can gain water, thereby decreasing the active material concentration.
In view of the foregoing, flow battery systems capable of managing the concentrations of various components in an electrolyte solution would be highly desirable in the art. The present disclosure satisfies the foregoing needs and provides related advantages as well.